340 hp capabilities?

Hopefully he is not the one who gave you the 600 hp number. Can it be done? Sure. Will it grenade before it makes it 8000 miles? That no one can tell you. Play it safe and give up a few HP for longevity. On the street 490 HP is gonna be just as fun as 590. Maybe more so.
600 hp isn't all that unrealistic out of a small block. Especially if it is bored 4.100 and stroked 4.125 and a good set of heads.